Toxoplasma gondii is a ubiquitous protozoan parasite that infects humans in various settings. The parasite is mainly acquired during childhood and adolescence [1]. In industrially developed, temperate climate countries, the prevalence of infection has declined over the last 30 years [1], with 10 to 50 percent of adults aged 15 to 45 years displaying serological evidence of past infection [2]. Much higher rates of infection (up to 80 percent) are found in the tropics in communities exposed to contaminated soil, undercooked meat, or unfiltered water [3-5].

Once a person is infected, the parasite lies dormant in neural and muscle tissue and will never be eliminated. Studies based in Europe and North America suggest that the large majority of immunocompetent humans are able to limit the spread of the parasite and the associated tissue damage, ensuring that it remains in its dormant form [3]. Inflammation of the retina and choroid (retinochoroiditis) is the most frequent, permanent manifestation of toxoplasmic infection. In Europe and North America, up to 1 percent of infected individuals eventually develop such lesions [6-9].

Evidence has been accumulating over the last 15 years that these findings are not applicable to parts of Latin America, where clinical manifestations of infection are much more common and more severe, probably because of the predominance of more diverse and more virulent parasite genotypes [10-12]. Toxoplasma gondii strains in Europe and North America belong to three distinct clonal lineages, type I, type II, and type III [13]. A comparison of European and Brazilian cohort studies of newborns identified by universal screening showed that eye lesions were larger and more numerous and more likely to impair vision in Brazilian cohorts [14].

In contrast to Europe and North America, acquisition of toxoplasmosis during childhood or adulthood in Brazil accounts for high levels of eye disease. In parts of Brazil, up to 20 percent of the population has toxoplasmic retinochoroiditis, resulting in high levels of visual impairment [15-18]. Toxoplasmosis is a leading cause of blindness in South America [19], but not in Europe or North America [20,21].

When toxoplasmic infection is acquired for the first time during pregnancy, infection can be transmitted to the fetus, resulting in congenital toxoplasmosis and associated neurological and ocular manifestations. Continued parasite proliferation and tissue destruction can occur within the fetal brain even after a marked maternal immune response, including maternal IgG production [22].

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